This invention relates to the field of motion control, and, in particular, discloses an improved servomechanism having exceptional accuracy.
The purpose of any servomechanism is to move an object according to a command signal. The motion may be rotational or linear. In servomechanisms of the prior art, the motion of the object does not exactly follow the waveform of the command signal. The most obvious source of error is time lag; the object cannot move instantaneously to the position required by the command signal.
Certain applications demand a servomechanism having great accuracy. One example of such an application is the control of the position of the tool in a special lathe for making shaped metal parts. Unless the servomechanism responds accurately to the command signal, the contour of the part will not be formed exactly as desired, and the part may thus be partially or totally defective. This invention is not limited to machine tool applications, however, but can be used in many other contexts.
It has been known, in the prior art, to compensate for the inherent time lag in a servomechanism by advancing the waveform of the command signal by a known small amount, the amount being sufficient to place the object in the desired position at the desired time. However, this prior art method is not satisfactory for high-precision applications, because the amount of time lag required varies due to changes in variables such as temperature and the friction of mechanical components. Even a relatively small error in the setting of a time lag value can cause unacceptably large tracking errors in the system.
Other errors can prevent a perfect match between the command signal and the response of the servomechanism. Imperfections in the system can cause differences in the amplitude of the command and response signals. Also, the servomechanism may introduce a bias, or offset, to the command signal, which bias causes errors in the positioning of the object.
The present invention provides a method and apparatus for automatically adjusting the command signal to minimize errors in tracking. The invention has general applicability to the adjustment of various parameters of the command signal, such as time advance, amplitude, offset, and others. The invention provides a means whereby a plurality of such parameters may be sampled and adjusted virtually simultaneously, wherein the adjustment of one parameter does not interfere with the adjustment of another. The invention thereby provides a servomechanism having an extremely close correspondence between the command and position signals.